Out-of-reach rewards elicit human-oriented referential communicative behaviours in family dogs but not in family pigs

Human-oriented referential communication has been evidenced not only in domestic but also in some wild species, however, the importance of domestication-unrelated species’ characteristics in the emergence of this capacity remains largely unexplored. One shared property of all species reported to exhibit referential communication is the efficient use of visual social signals. To assess the potential role of species-specific characteristics in the emergence of human-oriented referential communication, we compared similarly socialised companion animals from two domestic species: dogs, which rely heavily on conspecific visual social signals; and pigs, which do not. We used an out-of-reach reward paradigm with three conditions: both human and reward present, only human present, only reward present. Both species exhibited certain behaviours (e.g. orientation towards the human, orientation alternation between the human and the reward) more often in the human’s presence. However, only dogs exhibited those behaviours more often in the simultaneous presence of the human and the reward. These results suggest similar readiness in dogs and pigs to attend to humans but also that pigs, unlike dogs, do not initiate referential communication with humans. The ability to referentially communicate with humans may not emerge in mammals, even if domesticated companion animals, that lack certain species characteristics, such as efficient intraspecific visual communication.

Dog Project (https://familydogproject.elte.hu/). Before enrolment, we asked the owners about 23 the socialization background of the dogs to ensure that it was similar to that of pigs. Owners 24 provided a written consent form to permit their dogs and pigs to participate in the study, as well 25 as to publish their data and images.  31 We measured the following behaviours from the moment the owner sat down in the Test of all 32 Conditions except from the Test in the Food Condition in which we measured it after the E 33 closed the door (60 s in all cases).

Behavioural Variables
Definition Vocalization (duration, s): S is emitting any vocalizations (e.g., barks, whines for dogs, grunts for pigs). Measured in Test of each condition.
Occurrence of vocalization (binomial variable): Number of individuals emitting any vocalizations (e.g., barks, whines for dogs, grunts for pigs). Measured in Test of each condition.
Orientation to the boxes (duration, s): S orients its head to any of the boxes, from a static position or meanwhile moving, with or without establishing physical contact with any of the boxes. Measured in Test of "Owner Condition". In Test of "Food Condition" and "Food + Owner Condition" it is the sum of S's orientation to the correct box (the one containing the food) and the empty box, although orientation to the correct box highly correlates with orientation to any of the boxes (Food Cond.: rs = 1, N = 24, p < 0.001; Food+Owner Cond.: rs = 1, N = 24, p < 0.001).
Orientation to the owner (duration, s): S orients its head from a static position or meanwhile moving, with or without establishing physical contact with the owner. Measured in Test of "Owner Condition" and "Food+ Owner Condition".
Orientation to the door (duration, s) S orients its head to the door, from a static position or meanwhile moving, with or without establishing physical contact with it. Measured in Test of "Food Condition".
Interaction with the boxes (duration, s): S establishes physical contact with any of the boxes. Measured in Test of "Owner Condition". In Test of "Food Condition" and "Food + Owner Condition" it is the sum of S's interaction with the correct box (the one containing the food) and the empty box, although interaction with the correct box highly correlates with interaction with any of the boxes (Food Cond.: rs = 1, N = 24, p < 0.001; Food+Owner Cond.: rs = 1, N = 24, p < 0.001).
Interaction ratio with the boxes (value between 0 and 1): The ratio of interaction with the boxes out of the total time spent orienting towards the boxes. Measured in Test of all conditions.
Orientation-alternation (frequency): Orienting to any of the boxes in "Owner condition", and orienting to the correct box in Test of "Food + Owner condition" (S orients its head towards the box from a static position or meanwhile moving, with or without physical interaction) followed or preceded within a maximum of 3 s by orientating towards O.
Orientation-alternation with the door (frequency): Orienting to the correct box in Test of "Food condition" (S orients its head towards the correct box from a static position or meanwhile moving, with or without physical interaction) followed or preceded within a maximum of 2 s by orienting towards the door.   Table   S3.

Alternation setting 40
The effect of the alternation coding setting was analysed in Friedman Tests. Durbin-Conover 41 post hoc tests were performed for pairwise comparisons, and we controlled for multiple 42 comparisons by adjusting p-values using Holm's method.

43
The alternation setting contained the maximum length of the gap between two behaviours 44 (in seconds, the first number in the variable levels) and the maximum length of the two 45 consecutive behaviours (in seconds, the second number in the variable levels). We investigated 46 six alternation settings (11,12,22,23,32,33). Alternation setting had no effect on the pigs' 47 orientation alternation between the boxes and the owner in any of the conditions (all p > 0.053).

48
However, alternation setting affected the orientation alternation of the dogs in all conditions 49 (all p < 0.001). In the Food Condition, settings in which the maximum length of the gap between 50 two behaviours was 3 sec (32, 33) differed from the other settings (11, 12, 22, 23) (all p < 51 0.001). In the Owner Condition, settings in which the maximum length of the gap was 3 sec 52 (32, 33) differed from settings in which the gap was 1 sec (11, 12) (all p < 0.016). In the 53 Food+Owner Condition, settings in which the maximum length of the gap was 1 sec (11, 12) 54 differed from the settings in which the gap length was higher (22,23,32,33).

55
Due to that pigs' alternation did not differ under the investigated settings, we chose the 56 setting which we found the most relevant based on previous studies on the topic (3,4) and more 57 indicative of the dynamic aspect of a referential alternation.